window.onscroll= function(){
//        滚动条距离顶部距离
    var t = document.documentElement.scrollTop||document.body.scrollTop;
    var ImgID = document.getElementById("sco");
    var distance = ImgID.scrollTop;
    if(t > 120)
    {
        ImgID.style.marginTop = t - 120  + "px";
        img2.style.opacity=0.4;
        img2.style.filter='alpha(opacity:'+40+')'
    }
    else {
        ImgID.style.marginTop = 0 + "px";
        img2.style.opacity=1;
        img2.style.filter='alpha(opacity:'+100+')'
    }
}